Composites are widely used in restorative dentistry as filling and fixing materials or in orthodontics as cements. Composites are generally defined as a combination of two substances whose properties differ from those of the pure components. In the case of dental composites they are multi-substance systems which consist of an organic monomer or polymer matrix in which one or more fillers are incorporated.
The main function of the fillers in these cases is to improve the mechanical properties of a dental composite, such as e.g. strength, hardness or modulus of elasticity, reduce thermal expansion and polymerization shrinkage and influence the rheological and visual properties of the composites in a targeted way. The dental fillers used for this can be divided into purely organic and inorganic fillers or combinations thereof, wherein inorganic fillers are most often used. These can in turn be divided into oxidic and non-oxidic fillers. The oxidic fillers are then further classified as siliceous and non-siliceous fillers.
The siliceous fillers include ground glasses, such as e.g. barium silicate glasses, strontium silicate glasses, lithium-aluminium-silicate glasses and X-ray-opaque aluminium fluoro-silicate glasses which are used primarily in methacrylate resin-reinforced glass ionomer cements. The siliceous fillers also include pure silicon dioxide fillers which are likewise often used in dental materials. Mixed oxides based on silicon and zirconium oxide or core-shell systems are also known. These are used, alongside the reinforcing action, to increase X-ray opacity and adjust transparency, by adapting the refractive index according to the composition of the filler.
Non-siliceous fillers such as e.g. zirconium oxide, tantalum oxide, ytterbium trifluoride or yttrium oxide are used as X-ray-contrast media. Aluminium and titanium oxide often serve as opacifiers because of their high refractive index.
Dental fillers should as a rule be colourless, resistant in the oral environment and toxicologically acceptable. To improve the mechanical properties, the surfaces of the dental fillers which are e.g. contained in the widely used light-curing composites are functionalized with polymerizable silanes, such as e.g. (meth)acryloyloxyalkyltrialkoxy silanes. The introduced (meth)acrylate groups are then covalently bound to the polymer matrix during the curing of the composites by copolymerization.
Filling composites with self-adhesive properties are currently attracting increased interest (N. Moszner, U. Salz, Macromol. Mater. Eng. 292 (2007) 245-271). These are composites which, alongside the conventional crosslinker or diluting monomers, such as e.g. bis-GMA (2,2-bis[4-(2-hydroxy-3-methacrylo-yl-oxypropoxy)phenyl]propane) or UDMA (1,6-bis[2-methacrylo-yl-oxyethoxycarbonylamino]-2,4,4-trimethylhexane) or D3MA (deca-ne-dioldimethacrylate) or TEGDMA (triethylene glycol dimethacrylate), contain strongly acid acid monomers. Such acid monomers, such as e.g. also GDMP (glycerol dimethacrylate dihydrogen phosphate) or MDP (10-methacryloyloxydecyl dihydrogen phosphate) are able to mediate adhesion to dentine and enamel.
If the conventional glass fillers, such as e.g. barium silicate glasses, are used for self-adhesive composites, disadvantageous effects may result. For one thing, the acid monomers of the organic matrix may dissolve cations, e.g. Ba2+ ions, out of the particles of the glass fillers. As a result of this, salt may form and, linked to this, there may be a clear viscosity increase of the organic matrix or a perceptible thickening of the composite and thus a deterioration in storage stability. There is also the risk that the acid monomer molecules will become bound to the filler surface because of the reaction of the acid phosphate groups with the filler and are thus no longer available as adhesion promoter for the hard tooth substance.
According to the state of the art the coating of dental glass fillers has already been known for a relatively long time. Thus e.g. EP 0 047 971 B1 or CH 652 139 A5 describe the coating of dental filler particles with a cold-, light- or heat-curable plastic, e.g. based on dimethacrylate.
There are numerous publications on the coating of dental glass powders with homo- or heteropolysiloxanes or sol-gel products, i.e. hydrolytic condensates of different silanes and metal alkoxides, such as Zr, Ti or Al alkoxides or their mixtures (cf. S. Klapdohr, N. Moszner, Monatsh. Chem. 136 (2005) 21-45).
In U.S. Pat. No. 6,620,861 B1, e.g. dental fillers are described in which glass powder particles are coated with polysiloxanes.
The coating of the glass fillers in the aforementioned documents takes place for different reasons and is intended to lead to advantageous properties of the dental materials containing these glass fillers. However, the use of the glass fillers in self-adhesive composites is not described in the above documents, nor is their possible suitability for use in self-adhesive compositions the subject of the disclosed teachings.
U.S. Pat. No. 5,453,456 also relates, not to self-adhesive composites, but to glass ionomer cements and aluminium-fluorosilicate glass fillers contained therein. To improve the mechanical properties of the cured cement while preserving or increasing the desired release of fluoride ions, the aluminium-fluoro-silicate glass filler particles are provided with a coating containing ionic carboxyl groups and siloxy groups. To further improve strength and fracture toughness, the thus-treated aluminium-fluorosilicate glass fillers can optionally be treated with an additional organic compound. Additional organic compounds coming into consideration comprise an extensive list of monomeric, oligomeric and polymeric compounds, wherein unspecifically polymers of different types are named as polymers, e.g. polycondensates, polyadducts and polymerizates, including among others polyvinyl chloride. Preferred additional organic compounds contain ethylenically unsaturated groups and hydrophilic groups, such as, for example, ethylene glycol groups, and in all the embodiment examples in which a treatment with an additional organic compound is carried out, compounds of just this preferred type are preferably used. A coating of the aluminium-fluorosilicate glass filler particles with polyvinyl chloride is not described.